Magnetic field measurements may be used in downhole surveying operations to determine the direction of the Earth's magnetic field at a particular point (or survey station). Magnetic field measurements may also be used to locate subterranean magnetic structures, such as a nearby cased borehole. These techniques are often used, for example, in well twinning applications in which one well (the twin well) is drilled in close proximity to and often substantially parallel with another well (commonly referred to as a target well).
The magnetic techniques used to sense a target well may generally be divided into two main groups; (i) active ranging and (ii) passive ranging. In active ranging, the local subterranean environment is provided with an external magnetic field, for example, via a strong electromagnetic source in the target well. The properties of the external field are assumed to vary in a known manner with distance and direction from the source and thus in some applications may be used to determine the location of the target well. In contrast to active ranging, passive ranging techniques make use of a preexisting magnetic field emanating from magnetized components within the target borehole. In particular, conventional passive ranging techniques generally take advantage of magnetization present in the target well casing string. Such magnetization may be residual in the casing string due to magnetic particle inspection techniques that are commonly utilized to inspect the threaded ends of individual casing tubulars.
In commonly assigned U.S. Pat. No. 7,656,161 to McElhinney, a technique is disclosed in which a predetermined magnetic pattern is deliberately imparted to a plurality of casing tubulars. These tubulars, thus magnetized, are coupled together and lowered into a target well to form a magnetized section of casing string including a plurality of longitudinally spaced pairs of opposing magnetic poles. Passive ranging measurements of the magnetic field may then be utilized to survey and guide drilling of a twin well relative to the target well. The distance between the twin and target wells may be determined from various magnetic field measurements made in the twin well (as further disclosed in commonly assigned U.S. Pat. No. 7,617,049). These well twinning techniques may be utilized, for example, in steam assisted gravity drainage (SAGD) applications in which horizontal twin wells are drilled to recover heavy oil from tar sands.
While the above described method of magnetizing wellbore tubulars has been successfully utilized in well twinning applications, there is room for yet further improvement. For example, the MWD sensors used to make the magnetic ranging and magnetic surveying measurements are commonly deployed a significant distance behind the drill bit (e.g., 15 to 20 meters) in a non-magnetic section of the bottom hole assembly (BHA). Those of ordinary skill in the art will appreciate that such a deployment increases the time between cutting (drilling) and ranging at a particular location which in turn hinders an operator's ability to make timely steering decisions. In twin well drilling and intercept applications it is particularly desirable to reduce the time (latency) between cutting and ranging so that steering decisions may be made in a timely fashion thereby reducing the tortuosity of the drilling well.
One difficulty in deploying magnetic sensors at or near the drill bit is that the lower BHA is generally highly magnetic (particularly the drill bit and motor) and can therefore strongly interfere with magnetic ranging and surveying measurements. Notwithstanding such difficulties, there remains a need in the art to make at-bit or near-bit magnetic ranging measurements in various drilling operations.